// gcFlushBgCredit flushes scanWork units of background scan work
// credit. This first satisfies blocked assists on the
// work.assistQueue and then flushes any remaining credit to
// gcController.bgScanCredit.
//
// Write barriers are disallowed because this is used by gcDrain after
// it has ensured that all work is drained and this must preserve that
// condition.
//
//go:nowritebarrierrec
func gcFlushBgCredit(scanWork int64) {
if work.assistQueue.head == 0 {
// Fast path; there are no blocked assists. There's a
// small window here where an assist may add itself to
// the blocked queue and park. If that happens, we'll
// just get it on the next flush.
atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
return
}
scanBytes := int64(float64(scanWork) * gcController.assistBytesPerWork)
lock(&work.assistQueue.lock)
gp := work.assistQueue.head.ptr()
for gp != nil && scanBytes > 0 {
// Note that gp.gcAssistBytes is negative because gp
// is in debt. Think carefully about the signs below.
if scanBytes+gp.gcAssistBytes >= 0 {
// Satisfy this entire assist debt.
scanBytes += gp.gcAssistBytes
gp.gcAssistBytes = 0
xgp := gp
gp = gp.schedlink.ptr()
ready(xgp, 0)
} else {
// Partially satisfy this assist.
gp.gcAssistBytes += scanBytes
scanBytes = 0
// As a heuristic, we move this assist to the
// back of the queue so that large assists
// can't clog up the assist queue and
// substantially delay small assists.
xgp := gp
gp = gp.schedlink.ptr()
if gp == nil {
// gp is the only assist in the queue.
gp = xgp
} else {
xgp.schedlink = 0
work.assistQueue.tail.ptr().schedlink.set(xgp)
work.assistQueue.tail.set(xgp)
}
break
}
}
work.assistQueue.head.set(gp)
if gp == nil {
work.assistQueue.tail.set(nil)
}
if scanBytes > 0 {
// Convert from scan bytes back to work.
scanWork = int64(float64(scanBytes) * gcController.assistWorkPerByte)
atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
}
unlock(&work.assistQueue.lock)
}
// gcDrainN blackens grey objects until it has performed roughly
// scanWork units of scan work or the G is preempted. This is
// best-effort, so it may perform less work if it fails to get a work
// buffer. Otherwise, it will perform at least n units of work, but
// may perform more because scanning is always done in whole object
// increments. It returns the amount of scan work performed.
//go:nowritebarrier
func gcDrainN(gcw *gcWork, scanWork int64) int64 {
if !writeBarrier.needed {
throw("gcDrainN phase incorrect")
}
// There may already be scan work on the gcw, which we don't
// want to claim was done by this call.
workFlushed := -gcw.scanWork
gp := getg().m.curg
for !gp.preempt && workFlushed+gcw.scanWork < scanWork {
// This might be a good place to add prefetch code...
// if(wbuf.nobj > 4) {
// PREFETCH(wbuf->obj[wbuf.nobj - 3];
// }
b := gcw.tryGet()
if b == 0 {
break
}
scanobject(b, gcw)
// Flush background scan work credit.
if gcw.scanWork >= gcCreditSlack {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
workFlushed += gcw.scanWork
gcw.scanWork = 0
}
}
// Unlike gcDrain, there's no need to flush remaining work
// here because this never flushes to bgScanCredit and
// gcw.dispose will flush any remaining work to scanWork.
return workFlushed + gcw.scanWork
}
// dispose returns any cached pointers to the global queue.
// The buffers are being put on the full queue so that the
// write barriers will not simply reacquire them before the
// GC can inspect them. This helps reduce the mutator's
// ability to hide pointers during the concurrent mark phase.
//
//go:nowritebarrier
func (w *gcWork) dispose() {
if wbuf := w.wbuf1.ptr(); wbuf != nil {
if wbuf.nobj == 0 {
putempty(wbuf, 212)
} else {
putfull(wbuf, 214)
}
w.wbuf1 = 0
wbuf = w.wbuf2.ptr()
if wbuf.nobj == 0 {
putempty(wbuf, 218)
} else {
putfull(wbuf, 220)
}
w.wbuf2 = 0
}
if w.bytesMarked != 0 {
// dispose happens relatively infrequently. If this
// atomic becomes a problem, we should first try to
// dispose less and if necessary aggregate in a per-P
// counter.
atomic.Xadd64(&work.bytesMarked, int64(w.bytesMarked))
w.bytesMarked = 0
}
if w.scanWork != 0 {
atomic.Xaddint64(&gcController.scanWork, w.scanWork)
w.scanWork = 0
}
}
// gcDrainN blackens grey objects until it has performed roughly
// scanWork units of scan work or the G is preempted. This is
// best-effort, so it may perform less work if it fails to get a work
// buffer. Otherwise, it will perform at least n units of work, but
// may perform more because scanning is always done in whole object
// increments. It returns the amount of scan work performed.
//
// The caller goroutine must be in a preemptible state (e.g.,
// _Gwaiting) to prevent deadlocks during stack scanning. As a
// consequence, this must be called on the system stack.
//
//go:nowritebarrier
//go:systemstack
func gcDrainN(gcw *gcWork, scanWork int64) int64 {
if !writeBarrier.needed {
throw("gcDrainN phase incorrect")
}
// There may already be scan work on the gcw, which we don't
// want to claim was done by this call.
workFlushed := -gcw.scanWork
gp := getg().m.curg
for !gp.preempt && workFlushed+gcw.scanWork < scanWork {
// See gcDrain comment.
if work.full == 0 {
gcw.balance()
}
// This might be a good place to add prefetch code...
// if(wbuf.nobj > 4) {
// PREFETCH(wbuf->obj[wbuf.nobj - 3];
// }
//
b := gcw.tryGetFast()
if b == 0 {
b = gcw.tryGet()
}
if b == 0 {
// Try to do a root job.
//
// TODO: Assists should get credit for this
// work.
if work.markrootNext < work.markrootJobs {
job := atomic.Xadd(&work.markrootNext, +1) - 1
if job < work.markrootJobs {
markroot(gcw, job)
continue
}
}
// No heap or root jobs.
break
}
scanobject(b, gcw)
// Flush background scan work credit.
if gcw.scanWork >= gcCreditSlack {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
workFlushed += gcw.scanWork
gcw.scanWork = 0
}
}
// Unlike gcDrain, there's no need to flush remaining work
// here because this never flushes to bgScanCredit and
// gcw.dispose will flush any remaining work to scanWork.
return workFlushed + gcw.scanWork
}
// gcFlushBgCredit flushes scanWork units of background scan work
// credit. This first satisfies blocked assists on the
// work.assistQueue and then flushes any remaining credit to
// gcController.bgScanCredit.
//
// Write barriers are disallowed because this is used by gcDrain after
// it has ensured that all work is drained and this must preserve that
// condition.
//
//go:nowritebarrierrec
func gcFlushBgCredit(scanWork int64) {
if work.assistQueue.head == 0 {
// Fast path; there are no blocked assists. There's a
// small window here where an assist may add itself to
// the blocked queue and park. If that happens, we'll
// just get it on the next flush.
atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
return
}
scanBytes := int64(float64(scanWork) * gcController.assistBytesPerWork)
lock(&work.assistQueue.lock)
gp := work.assistQueue.head.ptr()
for gp != nil && scanBytes > 0 {
// Note that gp.gcAssistBytes is negative because gp
// is in debt. Think carefully about the signs below.
if scanBytes+gp.gcAssistBytes >= 0 {
// Satisfy this entire assist debt.
scanBytes += gp.gcAssistBytes
gp.gcAssistBytes = 0
xgp := gp
gp = gp.schedlink.ptr()
// It's important that we *not* put xgp in
// runnext. Otherwise, it's possible for user
// code to exploit the GC worker's high
// scheduler priority to get itself always run
// before other goroutines and always in the
// fresh quantum started by GC.
ready(xgp, 0, false)
} else {
// Partially satisfy this assist.
gp.gcAssistBytes += scanBytes
scanBytes = 0
// As a heuristic, we move this assist to the
// back of the queue so that large assists
// can't clog up the assist queue and
// substantially delay small assists.
xgp := gp
gp = gp.schedlink.ptr()
if gp == nil {
// gp is the only assist in the queue.
gp = xgp
} else {
xgp.schedlink = 0
work.assistQueue.tail.ptr().schedlink.set(xgp)
work.assistQueue.tail.set(xgp)
}
break
}
}
work.assistQueue.head.set(gp)
if gp == nil {
work.assistQueue.tail.set(nil)
}
if scanBytes > 0 {
// Convert from scan bytes back to work.
scanWork = int64(float64(scanBytes) * gcController.assistWorkPerByte)
atomic.Xaddint64(&gcController.bgScanCredit, scanWork)
}
unlock(&work.assistQueue.lock)
}
// gcAssistAlloc1 is the part of gcAssistAlloc that runs on the system
// stack. This is a separate function to make it easier to see that
// we're not capturing anything from the user stack, since the user
// stack may move while we're in this function.
//
// gcAssistAlloc1 indicates whether this assist completed the mark
// phase by setting gp.param to non-nil. This can't be communicated on
// the stack since it may move.
//
//go:systemstack
func gcAssistAlloc1(gp *g, scanWork int64) {
// Clear the flag indicating that this assist completed the
// mark phase.
gp.param = nil
if atomic.Load(&gcBlackenEnabled) == 0 {
// The gcBlackenEnabled check in malloc races with the
// store that clears it but an atomic check in every malloc
// would be a performance hit.
// Instead we recheck it here on the non-preemptable system
// stack to determine if we should preform an assist.
// GC is done, so ignore any remaining debt.
gp.gcAssistBytes = 0
return
}
// Track time spent in this assist. Since we're on the
// system stack, this is non-preemptible, so we can
// just measure start and end time.
startTime := nanotime()
decnwait := atomic.Xadd(&work.nwait, -1)
if decnwait == work.nproc {
println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc)
throw("nwait > work.nprocs")
}
// gcDrainN requires the caller to be preemptible.
casgstatus(gp, _Grunning, _Gwaiting)
gp.waitreason = "GC assist marking"
// drain own cached work first in the hopes that it
// will be more cache friendly.
gcw := &getg().m.p.ptr().gcw
workDone := gcDrainN(gcw, scanWork)
// If we are near the end of the mark phase
// dispose of the gcw.
if gcBlackenPromptly {
gcw.dispose()
}
casgstatus(gp, _Gwaiting, _Grunning)
// Record that we did this much scan work.
//
// Back out the number of bytes of assist credit that
// this scan work counts for. The "1+" is a poor man's
// round-up, to ensure this adds credit even if
// assistBytesPerWork is very low.
gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(workDone))
// If this is the last worker and we ran out of work,
// signal a completion point.
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait > work.nproc {
println("runtime: work.nwait=", incnwait,
"work.nproc=", work.nproc,
"gcBlackenPromptly=", gcBlackenPromptly)
throw("work.nwait > work.nproc")
}
if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
// This has reached a background completion point. Set
// gp.param to a non-nil value to indicate this. It
// doesn't matter what we set it to (it just has to be
// a valid pointer).
gp.param = unsafe.Pointer(gp)
}
duration := nanotime() - startTime
_p_ := gp.m.p.ptr()
_p_.gcAssistTime += duration
if _p_.gcAssistTime > gcAssistTimeSlack {
atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime)
_p_.gcAssistTime = 0
}
}
// gcAssistAlloc performs GC work to make gp's assist debt positive.
// gp must be the calling user gorountine.
//
// This must be called with preemption enabled.
func gcAssistAlloc(gp *g) {
// Don't assist in non-preemptible contexts. These are
// generally fragile and won't allow the assist to block.
if getg() == gp.m.g0 {
return
}
if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" {
return
}
retry:
// Compute the amount of scan work we need to do to make the
// balance positive. When the required amount of work is low,
// we over-assist to build up credit for future allocations
// and amortize the cost of assisting.
debtBytes := -gp.gcAssistBytes
scanWork := int64(gcController.assistWorkPerByte * float64(debtBytes))
if scanWork < gcOverAssistWork {
scanWork = gcOverAssistWork
debtBytes = int64(gcController.assistBytesPerWork * float64(scanWork))
}
// Steal as much credit as we can from the background GC's
// scan credit. This is racy and may drop the background
// credit below 0 if two mutators steal at the same time. This
// will just cause steals to fail until credit is accumulated
// again, so in the long run it doesn't really matter, but we
// do have to handle the negative credit case.
bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit)
stolen := int64(0)
if bgScanCredit > 0 {
if bgScanCredit < scanWork {
stolen = bgScanCredit
gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(stolen))
} else {
stolen = scanWork
gp.gcAssistBytes += debtBytes
}
atomic.Xaddint64(&gcController.bgScanCredit, -stolen)
scanWork -= stolen
if scanWork == 0 {
// We were able to steal all of the credit we
// needed.
return
}
}
// Perform assist work
systemstack(func() {
gcAssistAlloc1(gp, scanWork)
// The user stack may have moved, so this can't touch
// anything on it until it returns from systemstack.
})
completed := gp.param != nil
gp.param = nil
if completed {
gcMarkDone()
}
if gp.gcAssistBytes < 0 {
// We were unable steal enough credit or perform
// enough work to pay off the assist debt. We need to
// do one of these before letting the mutator allocate
// more to prevent over-allocation.
//
// If this is because we were preempted, reschedule
// and try some more.
if gp.preempt {
Gosched()
goto retry
}
// Add this G to an assist queue and park. When the GC
// has more background credit, it will satisfy queued
// assists before flushing to the global credit pool.
//
// Note that this does *not* get woken up when more
// work is added to the work list. The theory is that
// there wasn't enough work to do anyway, so we might
// as well let background marking take care of the
// work that is available.
if !gcParkAssist() {
goto retry
}
// At this point either background GC has satisfied
// this G's assist debt, or the GC cycle is over.
}
}
// gcDrain scans roots and objects in work buffers, blackening grey
// objects until all roots and work buffers have been drained.
//
// If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt
// is set. This implies gcDrainNoBlock.
//
// If flags&gcDrainIdle != 0, gcDrain returns when there is other work
// to do. This implies gcDrainNoBlock.
//
// If flags&gcDrainNoBlock != 0, gcDrain returns as soon as it is
// unable to get more work. Otherwise, it will block until all
// blocking calls are blocked in gcDrain.
//
// If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
// credit to gcController.bgScanCredit every gcCreditSlack units of
// scan work.
//
//go:nowritebarrier
func gcDrain(gcw *gcWork, flags gcDrainFlags) {
if !writeBarrier.needed {
throw("gcDrain phase incorrect")
}
gp := getg().m.curg
preemptible := flags&gcDrainUntilPreempt != 0
blocking := flags&(gcDrainUntilPreempt|gcDrainIdle|gcDrainNoBlock) == 0
flushBgCredit := flags&gcDrainFlushBgCredit != 0
idle := flags&gcDrainIdle != 0
initScanWork := gcw.scanWork
// idleCheck is the scan work at which to perform the next
// idle check with the scheduler.
idleCheck := initScanWork + idleCheckThreshold
// Drain root marking jobs.
if work.markrootNext < work.markrootJobs {
for !(preemptible && gp.preempt) {
job := atomic.Xadd(&work.markrootNext, +1) - 1
if job >= work.markrootJobs {
break
}
markroot(gcw, job)
if idle && pollWork() {
goto done
}
}
}
// Drain heap marking jobs.
for !(preemptible && gp.preempt) {
// Try to keep work available on the global queue. We used to
// check if there were waiting workers, but it's better to
// just keep work available than to make workers wait. In the
// worst case, we'll do O(log(_WorkbufSize)) unnecessary
// balances.
if work.full == 0 {
gcw.balance()
}
var b uintptr
if blocking {
b = gcw.get()
} else {
b = gcw.tryGetFast()
if b == 0 {
b = gcw.tryGet()
}
}
if b == 0 {
// work barrier reached or tryGet failed.
break
}
scanobject(b, gcw)
// Flush background scan work credit to the global
// account if we've accumulated enough locally so
// mutator assists can draw on it.
if gcw.scanWork >= gcCreditSlack {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
if flushBgCredit {
gcFlushBgCredit(gcw.scanWork - initScanWork)
initScanWork = 0
}
idleCheck -= gcw.scanWork
gcw.scanWork = 0
if idle && idleCheck <= 0 {
idleCheck += idleCheckThreshold
if pollWork() {
break
}
}
}
}
// In blocking mode, write barriers are not allowed after this
// point because we must preserve the condition that the work
// buffers are empty.
done:
// Flush remaining scan work credit.
if gcw.scanWork > 0 {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
if flushBgCredit {
gcFlushBgCredit(gcw.scanWork - initScanWork)
}
gcw.scanWork = 0
}
}
// gcDrain scans roots and objects in work buffers, blackening grey
// objects until all roots and work buffers have been drained.
//
// If flags&gcDrainUntilPreempt != 0, gcDrain returns when g.preempt
// is set. This implies gcDrainNoBlock.
//
// If flags&gcDrainNoBlock != 0, gcDrain returns as soon as it is
// unable to get more work. Otherwise, it will block until all
// blocking calls are blocked in gcDrain.
//
// If flags&gcDrainFlushBgCredit != 0, gcDrain flushes scan work
// credit to gcController.bgScanCredit every gcCreditSlack units of
// scan work.
//
//go:nowritebarrier
func gcDrain(gcw *gcWork, flags gcDrainFlags) {
if !writeBarrier.needed {
throw("gcDrain phase incorrect")
}
gp := getg()
preemtible := flags&gcDrainUntilPreempt != 0
blocking := flags&(gcDrainUntilPreempt|gcDrainNoBlock) == 0
flushBgCredit := flags&gcDrainFlushBgCredit != 0
// Drain root marking jobs.
if work.markrootNext < work.markrootJobs {
for blocking || !gp.preempt {
job := atomic.Xadd(&work.markrootNext, +1) - 1
if job >= work.markrootJobs {
break
}
// TODO: Pass in gcw.
markroot(job)
}
}
initScanWork := gcw.scanWork
// Drain heap marking jobs.
for !(preemtible && gp.preempt) {
// If another proc wants a pointer, give it some.
if work.nwait > 0 && work.full == 0 {
gcw.balance()
}
var b uintptr
if blocking {
b = gcw.get()
} else {
b = gcw.tryGet()
}
if b == 0 {
// work barrier reached or tryGet failed.
break
}
// If the current wbuf is filled by the scan a new wbuf might be
// returned that could possibly hold only a single object. This
// could result in each iteration draining only a single object
// out of the wbuf passed in + a single object placed
// into an empty wbuf in scanobject so there could be
// a performance hit as we keep fetching fresh wbufs.
scanobject(b, gcw)
// Flush background scan work credit to the global
// account if we've accumulated enough locally so
// mutator assists can draw on it.
if gcw.scanWork >= gcCreditSlack {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
if flushBgCredit {
gcFlushBgCredit(gcw.scanWork - initScanWork)
initScanWork = 0
}
gcw.scanWork = 0
}
}
// In blocking mode, write barriers are not allowed after this
// point because we must preserve the condition that the work
// buffers are empty.
// Flush remaining scan work credit.
if gcw.scanWork > 0 {
atomic.Xaddint64(&gcController.scanWork, gcw.scanWork)
if flushBgCredit {
gcFlushBgCredit(gcw.scanWork - initScanWork)
}
gcw.scanWork = 0
}
}
// gcAssistAlloc performs GC work to make gp's assist debt positive.
// gp must be the calling user gorountine.
//
// This must be called with preemption enabled.
//go:nowritebarrier
func gcAssistAlloc(gp *g) {
// Don't assist in non-preemptible contexts. These are
// generally fragile and won't allow the assist to block.
if getg() == gp.m.g0 {
return
}
if mp := getg().m; mp.locks > 0 || mp.preemptoff != "" {
return
}
// Compute the amount of scan work we need to do to make the
// balance positive. We over-assist to build up credit for
// future allocations and amortize the cost of assisting.
debtBytes := -gp.gcAssistBytes + gcOverAssistBytes
scanWork := int64(gcController.assistWorkPerByte * float64(debtBytes))
retry:
// Steal as much credit as we can from the background GC's
// scan credit. This is racy and may drop the background
// credit below 0 if two mutators steal at the same time. This
// will just cause steals to fail until credit is accumulated
// again, so in the long run it doesn't really matter, but we
// do have to handle the negative credit case.
bgScanCredit := atomic.Loadint64(&gcController.bgScanCredit)
stolen := int64(0)
if bgScanCredit > 0 {
if bgScanCredit < scanWork {
stolen = bgScanCredit
gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(stolen))
} else {
stolen = scanWork
gp.gcAssistBytes += debtBytes
}
atomic.Xaddint64(&gcController.bgScanCredit, -stolen)
scanWork -= stolen
if scanWork == 0 {
// We were able to steal all of the credit we
// needed.
return
}
}
// Perform assist work
completed := false
systemstack(func() {
if atomic.Load(&gcBlackenEnabled) == 0 {
// The gcBlackenEnabled check in malloc races with the
// store that clears it but an atomic check in every malloc
// would be a performance hit.
// Instead we recheck it here on the non-preemptable system
// stack to determine if we should preform an assist.
// GC is done, so ignore any remaining debt.
gp.gcAssistBytes = 0
return
}
// Track time spent in this assist. Since we're on the
// system stack, this is non-preemptible, so we can
// just measure start and end time.
startTime := nanotime()
decnwait := atomic.Xadd(&work.nwait, -1)
if decnwait == work.nproc {
println("runtime: work.nwait =", decnwait, "work.nproc=", work.nproc)
throw("nwait > work.nprocs")
}
// drain own cached work first in the hopes that it
// will be more cache friendly.
gcw := &getg().m.p.ptr().gcw
workDone := gcDrainN(gcw, scanWork)
// If we are near the end of the mark phase
// dispose of the gcw.
if gcBlackenPromptly {
gcw.dispose()
}
// Record that we did this much scan work.
//
// Back out the number of bytes of assist credit that
// this scan work counts for. The "1+" is a poor man's
// round-up, to ensure this adds credit even if
// assistBytesPerWork is very low.
gp.gcAssistBytes += 1 + int64(gcController.assistBytesPerWork*float64(workDone))
// If this is the last worker and we ran out of work,
// signal a completion point.
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait > work.nproc {
println("runtime: work.nwait=", incnwait,
"work.nproc=", work.nproc,
"gcBlackenPromptly=", gcBlackenPromptly)
throw("work.nwait > work.nproc")
}
if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
// This has reached a background completion
// point.
completed = true
}
duration := nanotime() - startTime
_p_ := gp.m.p.ptr()
_p_.gcAssistTime += duration
if _p_.gcAssistTime > gcAssistTimeSlack {
atomic.Xaddint64(&gcController.assistTime, _p_.gcAssistTime)
_p_.gcAssistTime = 0
}
})
if completed {
gcMarkDone()
}
if gp.gcAssistBytes < 0 {
// We were unable steal enough credit or perform
// enough work to pay off the assist debt. We need to
// do one of these before letting the mutator allocate
// more to prevent over-allocation.
//
// If this is because we were preempted, reschedule
// and try some more.
if gp.preempt {
Gosched()
goto retry
}
// Add this G to an assist queue and park. When the GC
// has more background credit, it will satisfy queued
// assists before flushing to the global credit pool.
//
// Note that this does *not* get woken up when more
// work is added to the work list. The theory is that
// there wasn't enough work to do anyway, so we might
// as well let background marking take care of the
// work that is available.
lock(&work.assistQueue.lock)
// If the GC cycle is over, just return. This is the
// likely path if we completed above. We do this
// under the lock to prevent a GC cycle from ending
// between this check and queuing the assist.
if atomic.Load(&gcBlackenEnabled) == 0 {
unlock(&work.assistQueue.lock)
return
}
oldHead, oldTail := work.assistQueue.head, work.assistQueue.tail
if oldHead == 0 {
work.assistQueue.head.set(gp)
} else {
oldTail.ptr().schedlink.set(gp)
}
work.assistQueue.tail.set(gp)
gp.schedlink.set(nil)
// Recheck for background credit now that this G is in
// the queue, but can still back out. This avoids a
// race in case background marking has flushed more
// credit since we checked above.
if atomic.Loadint64(&gcController.bgScanCredit) > 0 {
work.assistQueue.head = oldHead
work.assistQueue.tail = oldTail
if oldTail != 0 {
oldTail.ptr().schedlink.set(nil)
}
unlock(&work.assistQueue.lock)
goto retry
}
// Park for real.
goparkunlock(&work.assistQueue.lock, "GC assist wait", traceEvGoBlock, 2)
// At this point either background GC has satisfied
// this G's assist debt, or the GC cycle is over.
}
}
